Inhibitor-Stimulated Non-Parallel Pancreatic Secretion in Man: Hormonal and Neural Regulation?

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Abstract
We wanted to study whether a total inhibition of tryptic activity in the duodenum would induce a cholecystokinin (CCK)-dependent increase in pancreatic exocrine proteinase secretion. Concentrations of CCK and activities and concentrations of pancreatic enzymes were measured in human plasma and duodenal juices, respectively, collected during continuous intraduodenal instillations of proteinase inhibitors, with and without intravenous atropine administration. Inhibitor instillation totally abolished tryptic activity and reduced the chymotryptic and elastase (1 and 2) activities by 95-100%. The inhibitors caused a rapid increase in the concentrations of trypsin, chymotrypsin, and pancreatic secretory trypsin inhibitor (PSTI) but had only a slight or no effect on amylase and elastase 1 secretion. An enhanced secretion of PSTI lends support to a possible connection between PSTI (resembling the monitor peptide causing CCK release in rats) and the enzyme secretion in man. CCK increased from 7 to 12-13 pmol/l. Intravenous atropine almost completely blocked the inhibitor-stimulated enzyme and PSTI secretion and reduced amylase activity by 50%. A further significant (P = 0.002) increase in the inhibitor-induced CCK output was found during atropine administration, as compared with the test situation without atropine. The inhibitor-induced pancreatic secretion during total inhibition of tryptic activity shows a non-parallel secretion requiring different signals for different enzymes. The increase in plasma CCK levels indicates that CCK is feedback-regulated by both an inhibitor-mediated decrease in duodenal enzyme activity and a further decrease in pancreatic enzyme secretion by atropine.